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Heat Transfer and Melting in Subglacial Basaltic Volcanic Eruptions: Implications for Volcanic Deposit Morphology and Meltwater Volumes

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Heat Transfer and Melting in Subglacial Basaltic Volcanic Eruptions: Implications for Volcanic Deposit Morphology and Meltwater Volumes Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 Heat transfer and melting in subglacial basaltic volcanic eruptions: implications for volcanic deposit morphology and meltwater volumes LIONEL WILSON 1'2 & JAMES W. HEAD, III 2 1 Department of Environmental Science, Lancaster University, Lancaster LA1 4YQ, UK (e-mail: L. [email protected]) 2 Department of Geological Sciences, Brown University, Providence, RI 02912, USA Abstract: Subglacial volcanic eruptions can generate large volumes of meltwater that is stored and transported beneath glaciers and released catastrophically in j6kulhlaups. At typical basaltic dyke propagation speeds, the high strain rate at a dyke tip causes ice to behave as a brittle solid; dykes can overshoot a rock-ice interface to intrude through 20-30% of the thickness of the overlying ice. The very large surface area of the dyke sides causes rapid melting of ice and subsequent collapse of the dyke to form a basal rubble pile. Magma can also be intruded at the substrate-ice interface as a sill, spreading sideways more efficiently than a subaerial flow, and also producing efficient and widespread heat transfer. Both intrusion mechanisms may lead to the early abundance of meltwater sometimes observed in Icelandic subglacial eruptions. If meltwater is retained above a sill, continuous melting of adjacent and overlying ice by hot convecting meltwater occurs. At typical sill pressures under more than 300 m ice thickness, magmatic CO2 gas bubbles form c. 25 vol% of the pressurized magma. If water drains and contact with the atmosphere is established, the pressure decreases dramatically unless the overlying ice subsides rapidly into the vacated space. If it does not, further CO2 exsolution plus the onset of H20 exsolution has the potential to cause explosive fragmentation, i.e. a fire-fountain that forms at the dyke-sill connection, enhancing melting and creating another candidate pulse of meltwater. The now effectively subaerial magma body becomes thicker, narrower, and flows faster so that marginal meltwater drainage channels become available. If the ice overburden thickness is much less than c. 300m the entire sill injection process may involve explosive magma fragmentation. Thus, there should be major differences between subglacial eruptions under local or alpine glaciers compared with those under continental-scale glaciers. Subglacial volcanic eruptions have been studied processes elsewhere on Earth (Mathews 1947; extensively in Iceland (Bj6rnsson 1975; Allen Skilling 1994; Smellie & Skilling 1994; Chapman 1980; Gudmundsson & Bj6rnsson 1991; Gud- et al. 2000) and on Mars (Allen 1979; Hodges & mundsson et al. 1997; Johannesson & Saemunds- Moore 1994; Head & Wilson 2002). son 1998) due to the ongoing nature of the Dykes represent the propagation, both later- process and the many beautifully exposed land- ally and vertically, of sub-vertical magma-filled forms and deposits. Of particular interest is the cracks from crustal or subcrustal reservoirs into generation of large volumes of meltwater, its the surrounding area. Dykes may propagate storage and transport below the glaciers, and the to the surface to cause eruptions; may propa- catastrophic meltwater release at glacial margins gate to the near-surface to set up stress fields, to produce j6kulhlaups (Bj6rnsson 1975, 1992). which under suitable conditions result in graben Documentation of the products and landforms (Mastin & Pollard 1988; Rubin 1992); or may resulting from these eruptions (Bj6rnsson 1975; stall and cool in the crust at depths too great to Allen 1980; Gr6nvold & Johannesson 1983; produce visible indications of their presence. The Gudmundsson et al. 1997; Johannesson & Sae- latter includes the possibility that they may cease mundsson 1998) and continuing study of active vertical propagation at some relatively shallow examples (Gudmundsson et al. 1997), together depth and then spread sideways to produce sills. with the development of qualitative and quanti- This process is encouraged if the least principal tative models of the processes (Einarsson 1966; stress ceases to be horizontal and becomes Gudmundsson et al. 1997; H6skuldsson & vertical. The discontinuity in density and other Sparks 1997; Hickson 2000; Smellie 2000), has material properties provided by the contact led to the recognition of candidates for these between a glacier or ice-cap and the underlying From: SMELLIE, J. L. & CHAPMAN, M. G. (eds) 2002. Volcano-Ice Interaction on Earth and Mars. Geological Society, London, Special Publications, 202, 5-26. 0305-8719/02/$15.00 © The Geological Society of London 2002. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 6 L. WILSON & J. W. HEAD rocks may also be a trigger for such activity, and magma reservoirs are typically c. 3 MPa (Parfitt subglacial eruptions are likely to begin with the 1991) and for reservoir depths of a few kilo- intrusion of a sill at the rock-ice boundary. metres these correspond to similar pressure Commonly, a subaerial basaltic eruption is gradients of c. 1000 Pa m -1. The consequence is initially manifested as a curtain of fire along a that the magma in mafic dykes with typical fissure tens to hundreds of metres long which widths of c. 1 m propagates upward at speeds of marks the surface trace of the dyke. Cooling c. 1 m s -1 (Wilson & Head 1981). The strain rates along the narrow parts of the dyke (Wilson & near the dyke tips implied by these speeds are Head 1988) causes localization of extrusion c. 1 s -I, about seven orders of magnitude larger within a few hours to a few days, and transition than the strain rates at which the surrounding ice to a centralized vent eruption (Head & Wilson can flow plastically given the rheological models 1987; Bruce & Huppert 1989). In submarine (a pseudo-plastic power-law fluid with a yield (Head et al. 1996) and subglacial basaltic erup- strength) proposed by Glen (1952), Nye (1953) tions, a classical initial curtain of fire does not and Paterson (1994). Thus a dyke can easily generally occur because of the inhibition of gas overshoot an ice-rock interface because the ice exsolution due to the pressure of the overlying appears to the propagating crack as a brittle, water or ice. In submarine eruptions, the sup- low-density rock with elastic properties similar pression of gas release continues throughout the to those of the basalt substrate. We show that eruption, but in subglacial eruptions the situa- the amount of ice melting which takes place on tion may become much more complex. Melting the timescale of dyke emplacement may be small of the ice overlying the initial sill may form a enough for the emplacement process to be cavity. As long as the overlying ice does not stable, though subsequent, more extensive ice deform too quickly, the pressure in the cavity melting may lead to collapse of the dyke. may be less than the lithostatic load which acted The pressure distribution in a dyke propagat- on the sill during the early stages of the intrusion ing through an elastic medium is dictated by process, and this may lead to an increase in several requirements that must be met simulta- gas exsolution and magma vesiculation, possibly neously. Most fundamental is that the distribu- resulting in magma fragmentation and some tion of stress across the dyke wall (dictated by form of explosive activity. The overlying ice both the internal pressure distribution and the cover may be completely removed, exposing external stress distribution) must be such as to magma to the pressure of the atmosphere and hold open the sides of the fracture into which leading to more vigorous explosive activity. magma is moving. There must also be a vertical With suitable additions, existing physical pressure gradient in the magma to support the models for the ascent and eruption of magma static weight of the magma, and an additional (Wilson & Head 1981, 1983) can be applied to pressure gradient in the direction of magma subglacial environments. Here we develop some travel to drive the motion against wall friction. simple physical principles for the intrusion of To maximize the flow speed, and hence the mass magma into a glacial cover and assess the impli- and volume fluxes through a dyke of a given cations for eruption behaviour and the nature of shape, the pressure in the propagating tip of the the resulting volcanic deposits and meltwater dyke, Ppt, must decrease to a low value. The release processes. We discuss the conditions theoretical ideal tip pressure is zero, but Rubin under which hyaloclastites and lava breccias (1993) suggested that tip pressure would in fact form, and show how either lava flow units or sill- be no smaller than the pressure at which the like bodies can form at the base of the ice, most soluble volatile species which the magma depending on the melting rate and behaviour of contains, commonly water, becomes saturated. the ice dictated by its thickness. The argument is that if the pressure falls slightly below the value at which the magma is saturated Subglacial and englacial dyke emplacement in this volatile, more of the volatile exsolves. The solubility function for water in basalt (Wilson & Mafic dykes sourced in crustal magma reservoirs Head 1981) is: are driven upward by magma buoyancy, by the nw = KwP 07 (1) presence of an excess pressure in the reservoir, or by a combination of the two. We shall show in where the constant Kw is 6.8 × 10 -8 if nw is later sections that typical mafic magmas have expressed as a mass fraction and P is the bulk densities smaller than those of their host pressure in Pascals. If the magma contains 0.25 rocks by Ap =c. 200kgm -3, so that the buoy- mass% water, a plausible value for a mafic ancy pressure gradient acting on them (g Ap =) is magma (Gerlach 1986), n=0.0025 and the c.
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